1. Field of the Invention
This invention relates to semiconductor devices, and more particularly to monolithic microwave integrated circuit (MMIC) designs for improved manufacturability.
2. Discussion
Advances in semiconductor device technology have recently included the improvement in design and manufacturability of integrated devices and systems. For instance, one type of integrated device that recently has received increased attention is monolithic microwave integrated circuits (MMIC) for application in radar detection systems. Radar systems are often used in conjunction with munition and obstacle detection sensor systems for sensing electromagnetic radiation in the microwave frequency band. Specifically, the development of radar for future military defense systems will incorporate the use of electronically steered antennas (ESA) that offer improved beam agility, higher power and increased target range. The ESAs are comprised of an array of passive and active integrated circuits that transmit and receive the electronic radar signals. The transmit/receive (T/R) modules include microwave integrated circuits that are used by the thousands for each radar system and are a significant cost driver in the production of an affordable radar system.
In general, microwave integrated circuit devices are semiconductor devices fabricated by combining one or more semiconductor layers. Of the several conventional methods known, one method of fabricating microwave integrated circuits is to form a junction that includes a transition from a n-type (electron conduction) to a p-type (hole conduction) region. Typically, this can be accomplished by one or more methods such as formation of a junction by diffusion of dopants, ion implantation of dopants, or the growth of contiguous n-type and p-type layers. These methods, however, generally require the use of complex equipment and extensive processing steps. It follows then that the fabrication of typical microwave integrated circuit devices can be relatively expensive.
An alternative and relatively more simple junction formation technique involves forming a Schottky barrier, whereby a metal is deposited on a semiconductor layer. Because of some potentially adverse metal-semiconductor reactions, and sensitivities to surface conditions and small voltage steps obtainable particularly with n-type materials, the yield and quality of these devices has, until recently, been impractical for many microwave applications.
In recent years, advances in MMIC design technology, including the use of gallium arsenide (GaAs) as a semiconductor, have limited the use of conventional automated equipment in microwave circuit assembly facilities. Microwave circuit assembly is considered to be very complex because gallium arsenide integrated circuits are significantly smaller and more delicate than conventional silicon integrated circuits. It is believed that no automated high volume fabrication or assembly facility currently exists for gallium arsenide integrated circuits. However, despite the manufacturability disadvantages of selecting gallium arsenide in lieu of silicon or other materials as the semiconductor substrate, numerous advantages are also apparent. The major advantage being the gallium arsenide integrated circuits have faster switching speeds of logic gates and significantly lower parasitic capacitance to ground.
The mechanical properties of gallium arsenide are well below that of silicon in hardness, fracture toughness and Young's modulus. Gallium arsenide is very brittle, about one-half as strong as silicon. This means that a much greater degree of process control is mandated to ensure reliability and repeatability necessary to cost-effectively produce gallium arsenide MMIC.
Additionally, gallium arsenide MMIC technology requires that the electrical grounding paths be very short. Therefore, gallium arsenide wafer thinning is employed to reduce the thickness of MMIC wafers to approximately 0.004" to 0.010" thick. In comparison, conventional integrated circuits have a semiconductor wafer thickness in the range of 0.015" to 0.030". Following the wafer thinning processing, a through-substrate via etching process is then performed to form a ground path directly through the chip to circuitry loaded on the top of the MMIC surface. The top surface of the MMIC has electrical conductors that delineate circuitry capable of operating at microwave frequencies. In many cases, these conductors are made into structures called air bridge crossovers. Typically, air bridges are located at the field effect transistors (FETs) and at various capacitors located on the MMIC surface. Routinely, the air bridge crossovers are densely packed in close proximity on the MMIC top surface. These air bridge crossovers can be easily damaged and as such are not accessible to conventional high rate circuit assembly techniques.
In general terms, die-attach is the process of bonding an integrated circuit chip to a substrate to produce an electrical interface therebetween. Commonly utilized substrates include printed wire boards (PWB), thin film gold metallized alumina and multilayer alumina header packages. Conventional bonding mediums include electrically conductive epoxies or solder alloys selected from metals of the type including indium, lead, tin, gold, silver, platinum, palladium or combinations thereof. Moreover, solder die-attach is the process of metallurgically bonding the integrated circuit chip to the substrate or readout device. The metallurgical bond provides an electrical interface between the components and acts to dissipate heat during thermal operational cycling. Vias extending through the semiconductor wafer provide an electrical communication path between circuitry disposed on the top surface of the MMIC and the substrate.
The present invention is directed to an improved ground plane metallization layer provided on the bottom surface of a integrated circuit chip, and more preferably a MMIC chip, for solder attachment to a substrate. Until recently it was believed that completely filling the vias of gallium arsenide MMICs with solder during die-attach was desirable because the solder "post" could dissipate a greater amount of heat from components electrically interfaced with the vias. However, it has been discovered that solder-filled chip vias cause reliability problems during operational temperature cycling. The thermal coefficients of expansion of the solder alloy, the gallium arsenide MMIC chip and commonly employed substrates are not matched sufficiently to inhibit cracks from forming at the vias and propagating through the MMIC chip. In fact, in some instances the solder completely penetrates the via and flows onto the top surface of the MMIC. Such undesirable failure modes generate excessive scrap which has made application of gallium arsenide MMICs impractical.
Another disadvantage of conventional solder die-attach processing includes the excessively manual method of positioning solder preforms in accurate alignment between a MMIC chip and a substrate prior to reflowing the solder. Inaccurate positioning produces non-uniform interface layers which results in reduced electrical performance.
Among the advantages of the present invention is that relatively efficient die-attach processing of semiconductor devices, including microwave integrated circuits, can be realized without specialized equipment or handling requirements. The improved MMIC design is relatively inexpensive and can be carried out successfully utilizing standard solder die-attach technology. The present devices exhibit improved reliability in operation and in manufacturability as compared to conventional integrated circuits. Therefore, it is an object of the present invention to provide an improved MMIC backside metallization system which will preferentially "de-wet" the solder at the solder-via interface or bond-line. Such preferential "de-wetting" substantially increases MMIC reliability by eliminating failure modes associated with via cracking.